Then he let them practice a bit, and two more delighted boys there never was, as they sent the craft ahead up or down, starting and stopping her with a few turns of a wheel or lever.

“You may want to know how to run her some day in an emergency,” said Amos Henderson.  “No telling what will happen.”

“We hope nothing will,” spoke Jack.

“There’s no telling,” prophesied the inventor.

For several days the ship moved ahead at moderate speed.  The machinery, excepting for some minor accidents, worked smoothly.  The gas bag did not leak, which was the accident most dreaded, and it was not necessary to run the gas generator, which proved a saving of the valuable chemical from which the lifting-vapor was produced.

Now and then, when in need of water, the craft was lowered to the earth in a secluded spot near a stream or lake, and the tanks were filled for drinking and washing purposes.  But so far, from the time of the hasty flight, no one on the earth had spoken to the voyagers.  Nor, so far as was known, had their presence been noted, though the black speck in the sky might have furnished plenty of talk all over the country for those who observed it.  The weather was pleasant, but it was noticed that it was constantly growing colder.

One morning Jack, who was the first up, stuck his head out of the cabin door before he had finished dressing.  He quickly popped back again.

“Whew!” he exclaimed.  “Colder than Greenland!”

“What’s that about Greenland?” asked the professor, who had just awakened.

“It’s awful cold outside,” said Jack, shivering from the remembrance.

Without a word the professor, wrapping a dressing-gown about him, hurried to the engine room, where several thermometers were kept.  One was outside, and could be read through a glass side.

“No wonder you felt cold,” he said to Jack, when he returned.  “It’s ten degrees below zero!”

The boys hurried to complete their dressing.  The professor did likewise, as he was anxious to take some observations.

“Get out the fur garments,” he said.  “We must take no more chances now.  It will become colder rapidly, and ordinary clothes will be of no protection.”

The boys and the professor donned heavy fur coats, with immense gloves and caps that covered all of their faces but the eyes.  Then they went outside.  Jack was the first to look over the side of the ship.  As he did so he uttered a cry of astonishment.

Down below, about three-quarters of a mile, was a great white, snowy waste.  Giant mountains of ice were heaped on every side.  It was a cold, frosty silent world that the Monarch was flying over.  They had reached the frozen north!  They were at the beginning of the entrance to the land of the Pole!

CHAPTER X

LOST IN AN ICE CAVE

“I’m not surprised that the thermometer is down below zero,” remarked Jack.  “There’s enough ice under us to supply the whole United States.”